A touch sensor has been known as an interface for a user to input information to a display device. Arrangement of a touch sensor so as to overlap with a screen of a display device allows a user to input information and operate input buttons and icons displayed on the screen by directly or indirectly contacting the screen with a finger (a touch). For example, Japanese Patent Application Publication No. 2007-299409 discloses a touch sensor structured by a triangle electrode and a rectangular electrode arranged so as to cross a screen. In this touch sensor, one of the coordinates of a touch position is determined by using a gradient of an electric field generated by the triangle electrode.